An optical star coupler is one of important and indispensable optical devices in an optical fiber communication system, a light signal processing system, an optical measurement apparatus, an optical sensor, and the like, and has functions of dividing a light signal propagated through a light transmission path into N light signals (N is an integer equal to or greater than two) to be supplied to N light transmission paths, and combining N light signals propagated through N light transmission paths to provide a combined light signal to be supplied to a light transmission path. Among plural kinds of optical star couplers, a waveguide type of an optical star coupler is highly applicable to the above use, because it has advantages in mass-productivity, high reliability, economical feature, etc.
One type of a conventional optical waveguide type star coupler comprises plural Y-branching waveguides, each having one non-branching waveguide path and two divided waveguide paths, which are connected one after another, such that two divided waveguide paths of a Y-branching waveguide are connected to respective non-branching waveguide paths of two Y-branching waveguides which are positioned at a following stage to provide a multi-stage structure having one input/output port on one side and N output/input ports on the other side.
Another type of a conventional optical waveguide type star coupler comprises plural directional couplers which are also connected in the same manner. Such an optical star coupler is described in "Optical integrated circuits" published on Feb. 25, 1985 from Ohm-sha, Tokyo, Japan by Nishihara et. al. These conventional optical waveguide type star couplers will be explained in more detail later.
In a case where the optical waveguide type star coupler is used for an optical divider, an input light signal is supplied to the input port. Thus, the input light signal is sequentially divided into two light signals in each of the plural Y-branching waveguides which are connected to provide the multi-stage structure, so that N output light signals are obtained at the N output port.
However, the former conventional optical waveguide type star coupler has a disadvantage that a branching angle of each Y-branching waveguide which is required is less than three degrees to suppress a scattering loss of light. As a result, the entire length becomes large, thereby increasing an absorption loss of light in waveguide paths. In addition, the optical star coupler has an extremely elongated configuration, as the number N of the output ports is increased, so that a uniform distribution characteristic is difficult to be realized in view of fabricating processes such as a glass film formation process, a photolithography process, a dry-etching process, etc., although it has the advantage that a wavelength dependency of a branching ratio is small to provide a wide band characteristic.
The latter conventional optical waveguide type star coupler has also the disadvantage that a band is narrow and has a range of 50 to 100 .ANG. as compared to the former optical star coupler, and a branching ratio is largely changed by a structure and dimension deviation of a coupling portion of the directional coupler. In addition, a required branching angle of the directional coupler is also less than three degrees. As a result, the same disadvantage is resulted as explained in the former optical star coupler.